Wavelength division multiplexed (WDM) communication systems are known in which optical signals having different wavelengths are combined onto an optical path, including, for example, and optical fiber. Often, a so-called optical “add-drop multiplexer” (OADM) may be provided along the optical communication path, which demultiplexes or drops a sub-set of optical signals, while permitting other optical signals to continue propagating along the optical path (pass through optical signals). Other optical signals, which may have the same wavelength as those which were dropped may be or combined onto the optical path by the OADM, and such added signals may propagate along the optical path with the pass through optical signals.
Optical signals may suffer loss or attenuate during propagation along the optical path. Accordingly, one or more optical amplifiers, such as erbium-doped fiber amplifiers (EDFAs) and/or Raman amplifiers, may be provided along the optical path to boost or amplify the optical signals. In a WDM system including an OADM, the optical amplifiers may be provided upstream from the OADM to amplify optical signals input to the OADM, as well as downstream from the OADM, to amplify optical signals output from the OADM.
When a WDM system is installed or turned-up, certain optical signals may be added and/or dropped by an OADM, while optical amplifiers upstream from the OADM may not yet have been activated. In order to insure proper operation of the optical amplifiers, the optical signals applied thereto typically should have an adequate amount of optical power or exceed a threshold power. Below this threshold, the optical amplifier is unable to amplify an incoming optical signal. On the other hand, if pass through optical signals output from an optical amplifier upstream from an OADM have an excessive amount of optical power and are combined with optical signals added by the OADM, so-called “spiking” may occur, resulting in cross-talk between the pass through optical signal and the added optical signals, causing bit errors. Moreover, if such high power or “spiked” pass-through optical signals and the added optical signals are provided to an optical amplifier downstream from the OADM, the optical amplifier may allocate more gain to the pass through optical signals than to the added optical signals. The added optical signals, therefore, may not be amplified sufficiently, suffer excessive attenuation, and may not be adequately sensed at a receive end of the system.
A method and apparatus that eliminates or reduces “spiking,” while assuring proper operation of the optical amplifier in a WDM system, is, therefore, desired.